This invention relates, in general, to the manufacture of miniature size electric lamps which can be carried out simply and reliably with available automatic lamp manufacturing equipment. More particularly, the present invention deals with improved means to provide an electrical termination between the side-lead wire of a miniature lamp construction and the inner-surface of the metal lamp base.
Certain types of miniature size electric incandescent lamps, such as that commercially designated as the No. 387 sub-miniature lamp, are in general use at present for indicator and instrument lighting applications. Such lamps are customarily comprised of a tubular glass lamp bulb or envelope of very small size, eg, around 7/32 inch diameter, on one end which is mounted, axially of the bulb, a lamp base in the form of a metal shell of slightly larger diameter than that of the bulb and inflanged at its closed end so that the lamp can be inserted and pushed bulb end first into a lamp socket in an instrument or other socket-supporting panel. The bulb contains a filament connected across a pair of current lead-in wires which are hermetically sealed through the side wall of the bulb at the neck end thereof by a conventional butt-type seal. The seal or neck end of the bulb extends into an open end of and is secured to the base shell by a customary ring of lamp basing cement which is peripherally disposed in the ring around the inside wall of the base shell below the rim, and the wire leads are electrically connected to the metal shell and eyelet contact, respectively, of the lamp base. The open inner end of the base shell into which the bulb extends is formed with an inwardly rolled or inturned curved lip within which the bulb is snugly received to center it in place more or less axially of the base shell.
The earliest manner of electrically connecting the side wire lead of an electrical lamp to the base shell was to position the wire lead so as to extend out between the wall of the lamp bulb and the rim of the base shell when the base was placed in mounting position on the neck end of the bulb, and to then solder the wire lead to the outer side of the base shell adjacent its rim. Such procedure, however, resulted in a formation of a small mound or accumulation of the solder used for connection on the outer side of the base shell and protruding therefrom. For the intended usage of the particular sub-miniature indicator type lamps referred to above designed for bulb end first slide-in insertion of the lamp into the lamp socket, such protruding solder accumulations on the base shells would not be permissible since they would, in effect, block the endwise sliding insertion movement of the lamp into its socket. Accordingly, some other way of basing these type lamps had to be developed which would avoid the presence of any solder protrusions on the base shells.
To this end, the basing procedure now in customary use comprises the steps of forming the base shell with an apertured inward indent or recess in its side wall near the closed outer end of the base through the aperture of which indent the side wire lead is threaded and cut off flush with the outer side of the base shell, after which the cut-off wire lead is then soldered to the base shell within the indent therein. With this modified basing procedure, the solder accumulation at the wire-to-base shell solder connection is concealed entirely within the indent in the base shell and thus does not protrude from the outer side thereof. However, because of the necessity for threading the side wire lead through the side aperture in the base shell, such a modified basing procedure precludes the use of conventional type automatic base threading mechanisms similar to that disclosed in U.S. Pat. No. 2,120,877 -- Uber, for example, which are customarily employed in the lamp making art, because of the substantial cost saving realized therefrom, to automatically feed and position lamp bases in mounting position on the glass bulbs of electric lamps, with the end or center contact wire lead of the lamp bulb extending through the customary end contact eyelet of the base in position for soldering thereto. In their customary manner of operation, these automatic base-threading mechanisms lower the bases down over and guide the upstanding center contact wire lead, extending endwise from the lamp bulb, into and upwardly through the opening in the end contact eyelet of the base. Obviously, the presence of a side wire lead extending laterally from the lamp bulb and required to be threaded into a side aperture in the base shell of the lamp, as in the case of the modified lamp basing procedure referred to above, would interfere with the lowering movement of the lamp base shell, by such automatic base-threading mmechanisms, down over the upstanding center wire lead of the lamp and into mounting position on the end of the lamp bulb.
It has more recently been discovered in the manufacture of the larger size electric lamps that the side lead can be soldered directly to the inside surface of the lamp base shell at its rim. In this newer method, a portion of the side lead is flattened after which a quantity of solder is coated on the flattened lead-in portion, a flux applied over the solder and then the lamp envelope forcibly inserted into the base shell which mechanically wedges the side lead between the shell and the bulb neck. After the lamp envelope has been assembled in this manner to the base shell, the rim of the base shell is heated to melt the solder coating on the wire lead section to form a solder connection thereof to the base shell. Such a method is not particularly adapted to the high speed lamp manufacturing machines presently employed for miniature lamp construction by reason of the physical interference which takes place between the protruding side lead and the base shell which would either cause misalignment of the members being assembled or remove solder from the side lead and produce a poor electrical termination.
Many types of basing cements are also known to join a metal base to the glass envelope of an electric lamp. The original basing cements were electrically non-conductive and comprised of an inert filler material, an insulating binder, and a processing agent or vehicle. The typical binders were organic polymers which included shellac, phenol-aldehyde resins, silicon resins, and epoxy resins. More recently, it has been learned that conductive particulates, generally in the form of finely-divided metal powders, could be admixed to an otherwise electrically-insulating cement in order to render the entire composition electrically-conductive for certain lamp applications. A known lamp construction comprises a hermetically sealed glass envelope from which a pair of lead-in wires protrude, wherein the lead-in wires are adhesively bonded to the glass envelope by a conductive epoxy cement to separate metal tangs forming the lamp's circuit with a resistant filament connected between the lead-in wires.